The invention relates to an optical disc comprising a recording area for recording data encoded in optical marks at a recording density, the recording area comprising circular or spiral tracks provided with a servopattern comprising headers alternating with track portions, which headers comprise position information encoded at a header density, and which track portions comprise periodic characteristics.
The invention further relates to a recording device for recording data on the optical disc, which device comprises a recording head, recording control means and positioning means for positioning the recording head on a track at a position to be recorded, the positioning means comprising header detecting means for retrieving the position information from the headers.
The invention further relates to a read device for reading data from the optical disc, which device comprises a read head, read control means and positioning means for positioning the read head on a track at a position to be read, the positioning means comprising header detecting means for retrieving the position information from the headers.
Such a record carrier and apparatus are known from European Patent Application EP 0 587 019, document D1 in the list of related documents. The document discloses a record carrier in the form of an optical disc having a recording area comprising a pattern of grooves on a substrate, constituting a servopattern of circular or spiral tracks. The tracks are provided with periodic characteristics, constituted by servo pits, which are distributed along the turns of the track at regular angular intervals. The recording tracks are subdivided in longitudinal direction into track portions alternating with headers. The headers comprise position information, e.g. an address area comprising pre-recorded address marks. The headers are made during manufacture, e.g. in the form of so-called pre-pits formed by embossing. The address marks represent position information for positioning a recording head on a desired track and are indicative for the address of the recording area following the address area. In a direction transverse to the tracks, i.e. radially, the headers are aligned. The disc comprises one track address and a fixed number of radially aligned servo pits in each turn, constituting a so-called sampled servo pattern. The servo pattern comprising radially aligned elements is called Constant Angular Velocity (CAV) servo pattern, and is to be scanned by a servo system having a phase locked loop (PLL) to generate a servo frequency locked to the rotation frequency of the disc. The address marks are dimensioned to be read clocked by said servo frequency. Further a data phase locked loop is provided for generating a data clock locked to a speed of data read/write operations, which are performed at a substantially constant linear density, which corresponds to the well-known constant linear velocity (CLV) system. When jumping to a new radial position, the rotation frequency setting point or the data clock setting point are adjusted to the new position, but the servo phase locked loop remains locked to the CAV servo pattern. Hence the addresses in the headers can always be read at the servo frequency. The recording apparatus comprises an optical system for recording or reading information by generating a spot via a radiation beam on a track of the record carrier. The optical disc is rotated and the spot is positioned in radial direction on the centre of the track by servo means for scanning the track. During scanning the servo phase locked loop is locked to the rotation frequency of the disc for reading the CAV servo pattern. The data phase locked loop is locked to the CLV data speed. The known record carrier and apparatus have the problem, that for reliable detection of the headers a first phase locked loop must be locked to the CAV servo pattern, and a second phase locked loop must be locked to the CLV data density.
It is an object of the invention to provide an optical disc, a recording and a read device arranged for a more reliable and less complex detection of the headers.
For this purpose an optical disc as described in the opening paragraph is characterized according to the invention in that the phase of the periodic characteristics comprises a phase jump at a predetermined distance before each header. This has the effect, that the header detection means can be activated in time to reliably detect the headers, which follow the phase jump at said predetermined distance. This has the advantage, that errors in detecting headers, e.g. falsely detecting a header when scanning a track portion, are reduced, because the presence of the phase jump is to be detected first.
For the purpose mentioned above a recording device as described in the opening paragraph is characterized according to the invention in that the header detecting means are arranged for detecting a phase-jump in the phase of the periodic characteristics at a predetermined distance before each header. A read device as described in the opening paragraph is characterized according to the invention in that the header detecting means are arranged for detecting a phase jump in the phase of the periodic characteristics at a predetermined distance before each header. This has the advantage, that less errors occur in detecting headers in adverse reading conditions, e.g. in the presence of interference or dust.
The invention is also based on the following recognition relating to the reliability of the detection of headers in high density optical recording using CLV headers. The headers comprise marks, e.g. indicating the address, recorded at a header density, which in a CLV header pattern is substantially equal to the data recording density in the track portions. Hence, when scanning the disc, the headers cannot directly be detected from the frequency components of its content. However the periodic characteristics generate servo signals in a different frequency range, in which servo signals any stepwise change can be reliably detected separate from interference the data or address marks.
An embodiment of the optical disc is characterized in that the headers are radially aligned and the corresponding phase jumps are radially aligned. This has the advantage, that when scanning, cross talk from neighbouring tracks causes less disturbance of the servo signals.
Further advantageous, preferred embodiments of the apparatus and detection unit according to the invention are given in the dependent claims.